In order to avoid the dangers arising from the inequalities of the road, Stephenson so arranged the boiler of his new patent locomotive that it was supported upon the frame of the engine by four cylinders, which opened into the interior of the boiler. These cylinders were occupied by pistons with rods, which passed downwards and pressed upon the upper side of the axles. The cylinders opening into the interior of the boiler, allowed the pressure of steam to be applied to the upper side of the piston; and the pressure being nearly equivalent to one-fourth of the weight of the engine, each axle, whatever might be its position, had at all times nearly the same amount of weight to bear, and consequently the entire weight was pretty equally distributed amongst the four wheels of the locomotive. Thus the four floating pistons were ingeniously made to serve the purpose of springs in equalising the weight, and in softening the jerks of the machine; the weight of which, it must also be observed, had been increased, on a road originally calculated to bear a considerably lighter description of carriage. This mode of supporting the engine remained in use until the
progress of spring-making had so far advanced that steel springs could be manufactured of sufficient strength to bear the weight of locomotive engines.
The result of the actual working of the new locomotive on the improved road amply justified the promises held forth in the specification. The traffic was conducted with greater regularity and economy, and the superiority of the engine, as compared with horse traction, became still more marked. It is a fact worthy of notice, that the identical engines constructed in 1816 after the plan above described are to this day to be seen in regular useful work upon the Killingworth Railway, conveying heavy coal-trains at the speed of between five and six miles an hour, probably as economically as any of the more perfect locomotives now in use.
Mr. Stephenson’s endeavours having been attended with
such marked success in the adaptation of locomotive power to railways, his attention was called by many of his friends, about the year 1818, to the application of steam to travelling on common roads. It was from this point that the locomotive started, Trevithick’s first engine having been constructed with this special object. Stephenson’s friends having observed how far behind he had left the original projector of the locomotive in its application to railroads, perhaps naturally inferred that he would be equally successful in applying it to the purpose for which Trevithick and Vivian had intended their first engine. But the accuracy with which he estimated the resistance to which loads were exposed on railways, arising from friction and gravity, led him at a very early stage to reject the idea of ever applying steam power economically to common-road travelling. In October, 1818, he made a series of careful experiments in conjunction with Nicholas Wood, on the resistance to which carriages were exposed on railways, testing the results by means of a dynamometer of his own construction. The series of practical observations made by means of this instrument were interesting, as the first systematic attempt to determine the precise amount of resistance to carriages moving along railways. It was then for the first time ascertained by experiment that the friction was a constant quantity at all velocities. Although this theory had long before been developed by Vince and Coulomb, and was well known to scientific men as an established truth, yet, at the time when Stephenson made his experiments, the deductions of philosophers on the subject were neither believed in nor acted upon by practical engineers.
He ascertained that the resistances to traction were mainly three; the first being upon the axles of the carriages, the second, or rolling resistance, being between the circumference of the wheel and the surface of the rail, and the third being the resistance of gravity. The amount of friction and gravity he could accurately ascertain; but the
rolling resistance was a matter of greater difficulty, being subject to much variation. He satisfied himself, however, that it was so great when the surface presented to the wheel was of a rough character, that the idea of working steam carriages economically on common roads was dismissed by him as entirely impracticable. Taking it as 10 lbs to a ton weight on a level railway, it became obvious to him that so small a rise as 1 in 100 would diminish the useful effort of a locomotive by upwards of 50 per cent. This was demonstrated by repeated experiments, and the important fact, thus rooted in his mind, was never lost sight of in the course of his future railway career.
It was owing in a great measure to these painstaking experiments that he early became convinced of the vital importance, in an economical point of view, of reducing the country through which a railway was intended to pass as nearly as possible to a level. Where, as in the first coal railways of Northumberland and Durham, the load was nearly all one way,—that is, from the colliery to the shipping-place,—it was an advantage to have an inclination in that direction. The strain on the powers of the locomotive was thus diminished, and it was easy for it to haul the empty waggons back to the colliery up even a pretty steep incline. But when the loads were both ways, he deemed it of great importance that the railroad should be constructed as nearly as possible on a level.
These views, thus early entertained, originated in Stephenson’s mind the peculiar character of railroad works as distinguished from other roads; for, in railways, he early contended that large sums would be wisely expended in perforating barriers of hills with long tunnels, and in raising the lower levels with the excess cut down from the adjacent high ground. In proportion as these views forced themselves upon his mind and were corroborated by his daily experience, he became more and more convinced of the hopelessness of applying steam locomotion to common roads; for every argument in favour of a level railway was, in his